1. Field of the Invention
This invention relates to a multiplex communication method for use in a vehicle such as a motorcycle.
2. Related Art
It is preferred that a transmitter and receiver system to be installed on a vehicle should be simple in construction and lightweight. For this reason, it has been proposed a multiplex communication system of the type in which various transmitter and receiver units (or nodes) can be connected together through a single communication line so as to reduce the overall weight of the communication system.
FIG. 1 schematically shown such a multiplex communication system adapted to be installed on a vehicle 100 such as a motorcycle. The multiplex communication system in the form of a transceiver system comprises a radio receiver unit 101, an air suspension unit 102 for adjusting a road clearance of the vehicle 100, a first radio receiver control switch unit 103 to be operated by a driver or rider of the vehicle 100 for controlling the operation of the radio receiver unit 101, an air suspension control switch unit 104 to be operated by the rider for controlling the operation of the air suspension unit 102, a second radio receiver control switch unit 105 to be operated by a passenger of the vehicle 100 for controlling the operation of the radio receiver unit 101, and an indicator unit 106 indicating the condition of the operation of the radio receiver unit 101 as well as the condition of the operation of the air suspension unit 102.
The above units 101 to 106 have their respective input/output terminals 101a to 106a which are all connected to a common signal line or communication line 110. The transmission of information from one of the above units 101 to 106 to another is effected only through the common signal line 110. Thus, the multiplex communication system can be regarded as a LAN (Local Area Network) for effecting a mutual data transmission from one unit to another.
The above units 101 to 106 have digital microcomputers 111 to 116, respectively, each of which comprises a well-known transceiver section for serially outputting and receiving data signal DS for communication. As shown in FIG. 2, an output terminal 111a of the transceiver section of the microcomputer 111 contained in the radio receiver unit 101 is connected to a base of a transistor Q.sub.1 via a resistor R.sub.1-1, and a collector of the transistor Q.sub.1 is connected to the single signal line 110 via the input/output terminal 101a of the unit 101 and is also connected to a source of positive voltage Vcc via a resistor R.sub.2-1. An input terminal 111b of the transceiver section of the microcomputer 111 is connected to the collector of the transistor Q.sub.1, and an emitter of the transistor Q.sub.1 is grounded. The other units 102 to 106 are of the same construction as the unit 101.
The operation of the common signal line 110 will now be described.
When the signal at any one of the output terminals 111a to 116a of the microcomputers 111 to 116 is at a high level (H), the signal line 110 is grounded via the transistor Q of one of the units 101 to 106, so that the voltage of the signal line 110 is at a low level (L). And, when the signals at the output terminals 111a to 116a of the microcomputers 101 to 106 are all low (L), the voltage of the signal line 110 is high (H). Thus, with respect to the signal line 110, the low signal level is given a priority over the high signal level. More specifically, when the signal at any one of the input/output terminals 101a to 106a goes low (L), the voltage level of the signal line 110 is caused to go low (L) regardless of the state of the other input/output terminals.
The data signal DS sent through the signal line 110 will now be described. The data signal DS on the signal line 110 is of the start-stop type, and a format of such data signal DS is shown in FIG. 3. More specifically, the data signal DS includes a low-level start signal STR of 5.5 bits (in the case of the first start signal STR1), a start bit SB composed of a high-level signal or data D of 0.5 bit followed by a low-level gap G of 0.5 bit, 8-bit data portion (data bits), a parity bit PB composed of a parity signal or data D of 0.5 bit followed by a gap G of 0.5 bit, and a stop signal STP of 1 bit. Thus, the data signal DS is composed of 16.5 bits. The data portion may contain address data or text data, as later described, and each bit of the data portion is composed of 0.5-bit data D and 0.5-bit gap G. In this case, when the data D goes high, the gap G goes low, and vice versa. The reason why each bit of the data portion has 0.5-bit gap G is to enable an instantaneous determination of whether the signal line 110 is busy and an instantaneous determination of whether any collision of signals occurs on the signal line 110, that is to say, access to the signal line 110 is effected by more than one of the units 101 to 106 at the same time, as later described. Thus, the signal levels of the forward and rearward half portions D and G of each of the start bit SB, data bits and parity bit PB are in inverted relation to each other. When another one or more data signals DS are outputted onto the signal line 110 subsequently, these subsequent data signals DS has respective start signals STR of 1 bit (STR2, STR3, . . . ), so that each subsequent data signal DS is composed of 12 bits.
Thus, when the data signal DS is outputted onto the signal line 110, that is to say, the signal line 110 is busy, a time period of the high voltage level (H) of the signal line 110 is one bit time at the maximum. Therefore, whether the signal line 110 is busy can be determined by determining whether the time period of the high voltage level of the signal line 110 is not less than 1.5-bit time. The data portion of the first data signal DS1 contains a destination address data D-ADD representative of an address of the one of the units 101 to 106 which should receive the data signal, and a source address data S-ADD representative of an address of the one of the units 101 to 106 which transmitted the data signal.
The radio receiver unit 101 and air suspension unit 102 which process a larger amount of data than the other units 103 to 106 serve as master units which dominate the transmission of the data signal from the other units 103 to 106. Thus, the units 103 to 106 serve as slave units. And, the communication between a pair of units is effected through the signal line 110 according to one of the following two procedures:
Transmission of data from a master unit to a slave unit is carried out in accordance with the first procedure of communication. FIG. 4(a) shows a format of the 1st, 2nd, 3rd, . . . and Nth data signals DS1, DS2, DS3, . . . DSn successively transmitted from the master unit 101, 102 to the slave unit or indicator unit 6, and these data signals DS are sent unilaterally to the indicator unit 6. As shown, the data portion of the first data signal DS1 contains the address data, and the data portions of the second to nth data signals DS2 to DSn contain text data DATA. This transmission is hereinafter referred to as "asynchronous communication".
Referring to the second procedure of communication, the master unit 101 controls the transmission of the data signal DS from the slave units 103 and 105 while the master unit 102 controls the transmission of the data signal DS from the slave unit 104. More specifically, first, the data signal DS or request signal to be transmitted to the slave unit or destination unit 103, 104, 105 and having a data format of FIG. 4(b) is outputted onto the signal line 110 by the master unit or source unit 101, 102. The data portion of the request signal DS contains the address data composed of destination address D-ADD and source address S-ADD. Then, the microcomputer of the destination slave unit detects the coincidence of the destination address D-ADD, contained in the request signal DS sent from the source or master unit, with the address thereof, and this destination slave unit transmits the successive response data signals DS1, DS2 . . . DSn, which has a data format of FIG. 4(c), to the master unit or source unit designated by the source address contained in the above request signal. As shown, the data portions of the data signals DS1 to DSn contain text data DATA. This communication is hereinafter referred to as "synchronous communication". Thus, the master unit 101 controls the transmission of the data signal DS from the slave units 103 and 105, and the master unit 102 controls the transmission of the data signal DS from the slave unit 104. Therefore, the data signals are not freely sent to the master units 101 and 102 from their respective slave units, so that the processing of the master units 101 and 102 is not interrupted by the data signals from the slave units 103 to 105.
A collision of data on the signal line 110 occurs when more than one units send the respective data simultaneously. A mode of operation of avoiding such a data collision on the signal line 110 will now be described.
For example, as shown in FIG. 5, if the two master units 101 and 102 simultaneously send the data signal DS shown in FIG. 5(b) and the data signal DS shown in FIG. 5(c), respectively. With this system, when it is detected that the data signal DS outputted from any one of the units does not coincide in signal level with the data on the signal line 110, it is determined that a data collision occurs on the signal line 110. The two data signals DS outputted respectively from the master units 101 and 102 are equal in signal level to the signal on the signal line 110 until time t.sub.0. A time t.sub.0, the data signal DS from the master unit 101 is at the low level and is therefore given a priority over the data signal DS from the master unit 2 which is now at the high level, so that the voltage level of the signal line 110 is rendered low. Therefore, the master unit 102 detects the non-coincidence of the data signal sent therefrom with the data signal on the signal line 110, thereby deciding a collision of data on the signal line 110. As a result, the master unit 102 ceases to send the data signal while the master unit 101 continues to send the data signal. Such a data collision with respect to the master unit 101 is detected in the same manner.
Thus, the data collision is avoided, so that a data transfer among a plurality of units can be carried out via the single signal line or communication line.
In the multiplex communication system described above, when the data is to be received by one of the units or nodes, it is determined that such reception data is proper if the start signal STR and the stop signal ST are properly detected and if no parity errors exist in the reception data. Therefore, for example, if the forward half portions of an even number of bits of the signal data are inverted in signal level due, for example, to noises or the like, the parity error is not detected, and as a result it is erroneously determined that such reception data is correct.
Further, in the aforesaid multiplex communication system, when the units or nodes do not send the data signal onto the signal line, it can not be determined whether the unit actually does not need to transmit the data signal or the data-transmitting function of the unit is improper. Thus, such a malfunction of the unit can not be detected easily.